There are numerous known methods for recovering oil from vegetable materials. One technique in commercial use involves continuously pressing the vegetable material at low moisture content to expel oil. A pretreatment steaming of the vegetable material is frequently employed to facilitate the pressing separation.
Although mechanical pressing is a relatively simple procedure, it removes only part of the oil from the vegetable material. For this reason, a combination of continuous screw pressing followed by solvent extraction of the pressed meal is frequently employed on high oil-content seed materials, i.e., those containing more than 25% oil.
One disadvantage of the prepressing-solvent extraction technique for processing high oil-content materials is the high cost of the equipment. Not only must continuous screw presses be purchased and maintained but also a full-scale solvent extractor must be installed. Furthermore, it is usually necessary to flake the pressed cake to give particles that will hold together during solvent extraction, adding expense to the process.
An additional problem with the prepressing-solvent extraction technique relates to oil quality. That oil which remains in the pressed cake has been exposed to the air while at elevated temperatures. As a result, the oil extracted from the pressed cake is dark colored and difficult to refine to a light colored oil.
For the foregoing reasons, direct solvent extraction processes without mechanical pressing have been investigated. These processes are of two main types. One may be called a "percolation" process, wherein a basket or other liquid-permeable container is filled with the vegetable material and the extraction liquid is passed through it. Such containers are usually stacked in a vertical tower, with the solvent percolating downwards successively through each container in the stack and with the material acting as a self-filtering medium. Complete extraction of the oil by this process is difficult if the solid material becomes soggy and tends to pack, preventing a uniform permeation of the vegetable material by the solvent. In addition, if the solid to be extracted contains any appreciable amount of fine material, the fines may wash through with the solvent and contaminate the oil. The percolation process is also time-consuming since the solid must be contacted with solvent for long periods of time to obtain complete oil extraction.
A second type of extraction process may be characterized as a total immersion process. In such a process, the vegetable material is completely submerged in the extraction solvent. Oil is extracted more rapidly by this process. However, in order for such a process to be successful, the vegetable material must be finely divided and the solid must be agitated in order to provide good contact between the solid and the solvent. Such a process has heretofore been relatively unsatisfactory because appreciable amounts of finely-divided material have tended to remain in the miscella and thus be carried away by the miscella leaving the extraction unit. The crude oil must then be filtered before it can be further processed. The fines tend to clog the filters making their removal a difficult and expensive step.
We have now discovered a total immersion process which avoids the cost and problems of the expelling-extraction process and does not produce fines in the miscella. It is particularly suitable for the extraction of oil from high oil-bearing seeds, i.e., those containing more than 25% oil, from which oil is usually obtained by the expelling-extraction procedure.